Typically, a known two-stage supercharging system is provided with a high-pressure stage supercharger having a high-pressure stage turbine configured to be driven by exhaust gas discharged from an exhaust manifold, and a low-pressure stage supercharger having a low-pressure stage turbine being disposed downstream of the high-pressure stage supercharger in an exhaust system and configured to be driven by exhaust gas discharged from the high-pressure stage supercharger. Such a two-stage supercharging system has been employed in an automobile diesel engine or the like in particular.
In the mid or low speed operational range of an engine, performing two-stage supercharging (complete two-stage supercharging) in which both of the high-pressure stage supercharger and the low-pressure stage supercharger are driven is advantageous in terms of improvement of the low-speed torque and the transient property of the engine. On the other hand, in the high speed operational range of an engine, performing one-stage supercharging with the low-pressure stage supercharger with exhaust gas bypassed from the high-pressure stage supercharger makes it possible to achieve a higher compressor efficiency, thereby realizing stable operation with flexible matching.
Furthermore, in the intermediate range between the above described full two-stage supercharging and the one-stage supercharging, variable two-stage supercharging is performed, in which the low-pressure stage supercharger is driven while controlling the flow rate of exhaust gas supplied to the high-pressure stage supercharger to control driving of the high-pressure stage supercharger. In this variable two-stage supercharging, the opening degree of a high-pressure stage turbine bypass valve (exhaust flow control valve) disposed between the exhaust manifold and the high-pressure stage supercharger is regulated, and thereby the flow rate of exhaust gas supplied to the high-pressure stage supercharger is changed in accordance with the target output of the engine.
Furthermore, in the high-speed operational range of the engine, if there is a risk of overboost of the engine, a control is performed to regulate the opening degree of a low-pressure side turbine bypass valve (waste-gate valve) disposed between the high-pressure stage supercharger and the low-pressure stage supercharger to reduce the flow rate of exhaust gas supplied to the low-pressure stage supercharger.
As described above, typically, a flap type valve has been often employed as an exhaust-flow-rate control valve such as the high-pressure stage turbine bypass valve and the low-pressure stage turbine bypass valve, as disclosed in Patent Document 1 for instance.